Moisture accumulation on and around lumber is known to cause wood rot and the only cure is simply to replace the rotted wood with fresh wood. The problem is most acute in exterior wooden structures, in particular wooden decks, balconies, and other wooden structures which are erected out of doors and are exposed to frequent soaking moisture accumulations. Rain, snow melt, and simply humidity can all cause accumulations of moisture on and in such structures, leading to rotting of the wood. The problem is further added to by the lack of air circulation between the two portions of lumber where they intersect.
The problem can also occur however in interior wooden structures, particularly in roof joists and basement joists. The problem occurs most often where two portions of lumber are located and fastened across one another with one overlying and crossing the other, although any lumber exposed to moisture which does not readily run off will eventually rot.
Thus in exterior decks and balconies, and stairs or steps, joists are usually erected at appropriate spacings, typically about twelve to sixteen inches apart. Decking lumber is then nailed across the joists typically, although not exclusively, at right angles. The decking lumber is usually spaced apart by not more than one quarter of an inch. This allows rain and snow melt to drip down off the deck lumber through the gaps between adjacent portions of deck lumber and allows the surface of the deck lumber to dry out. However where the deck lumber intersects or crosses over the joists, moisture will inevitably penetrate between the deck lumber and the joists. This moisture is then trapped and cannot run off in the usual way. Further, due to the contact between the two portions of deck and joist lumber no air circulation can take place and this delays or entirely prevents the drying of the two portions of lumber at each intersection. The joist is then simply soaking up the moisture and this will cause rotting of the joist beneath the overlying deck lumber. In some cases the accumulation of moisture in these locations can also cause rotting of the underside of the deck lumber as well as rotting of the joists. Even where the two portions of lumber are closely bonded together, capillary action of moisture will cause moisture to seep in between them. This moisture is then trapped and remains there long after the upper or exposed surfaces of the lumber have dried out. The problem is somewhat aggravated by the nature of the nails fastening the deck lumber to the joists. When nails pass through the deck lumber and penetrate the top of the joists the nails cause depressions in the joists around the nails. Any moisture entering between the two portions of lumber of drawn in by capillary action is then trapped between the deck lumber and the joists will thus tend to collect in these depressions and will gradually seep down into the joists around the nails. The same moisture can be drawn upwardly into the underside of the deck lumber, again by capillary action similar to the action of a wick. The more the upper surface of the deck lumber dries out, the greater will be the wick action of the wood drawing the moisture upward.
This problem is aggravated by the area of contact between each portion of deck lumber and the underlying joists. Most deck lumber has a nominal width of four inches, and the joists, which are on edge, in most cases have a nominal width of two inches.
The area of contact at each crossing or intersection is thus nominally two inches by four inches making a total nominal contact area of eight square inches. This considerable area at each crossing thus provides a substantial area for accumulations of moisture, and also the prevention of air circulation.
The problem of moisture accumulation can also occur to a lesser extend on the upper edges of the joists where they are exposed in the gaps between the deck lumber. This is caused by the fact that it is desirable from the aspect of the users of the deck to place the portions of deck lumber as close as possible to one another, allowing only a minimum of spacing between the deck lumber portions for water drainage. Any noticeable spacing between the portions of deck lumber may damage shoes, and may permit small objects to fall through and be lost underneath the deck. The spacing is therefore minimised.
Run off moisture can accumulate on the small lengths of joists exposed between the portions of deck lumber, and due to the very small spacing, in most cases, it will eventually start to cause rot in these locations also. In addition moisture falling in these areas on the exposed lower timbers will tend to be drawn in under the adjacent overlying upper timbers a cause rot in these areas.
Clearly it is desirable to avoid the accumulations of moisture on the joists and under the deck lumber, and to provide a simple means for causing run off of rain and snow melt, in particular, from the deck, without accumulating on the joists. It is also advantageous if the contact area between the upper deck portions and lower joist portions of lumber can be reduced. This will reduce the area available for moisture accumulation, and thus reduce the quantity of moisture which can be trapped at any given location. This will greatly speed up the drying out process. Further it is desirable to provide for air circulation through the intersections so as to again speed up the drying out process. The same factors are true for many outdoor structures where lumber is exposed to weathering and moisture, and for certain indoor structures as well. It is also desirable to provide some means for preventing migration of moisture from the lower timbers where they are exposed to falling moisture back under the adjacent overlying timbers.
At first sight it would appear to be fairly straightforward to provide a cap of some moisture proof material to lay over the upper edges of the joists. In the past there has been a proposal to provide a solution to a somewhat related problem. For example U.S. Pat. No. 559,194, title Means For Protecting Foundation Timbers from Rot, issued in 1896, proposed dealing with a problem caused by moisture ascending up building foundation piles, due to capillary action, which would then damage the underside of the foundation timbers laid on top of the piles. This is the opposite of downward drainage.
The solution proposed was to provide metal caps which were to be nailed to the underside of the timbers. These caps would then overlie the tops of the building piles, and prevent moisture from ascending up the piles and rotting the foundation timbers by moisture entering from below. However this proposal did not deal with the problem of downward flow of moisture accumulating on the under surface of the lumber. In fact it increased this problem because the metal caps had to have an area greater than the area of the tops of the foundation pilot. This would then provide a greater area of moisture entrapment on the underside of the timbers or joists.
Thus while this proposal might have prevented upward flow of water around the foundation piles it did so at the expense of increasing the problems, described above, caused by downward flow of rain and snow melt which flows down between the deck lumber and accumulates on the upper surface of the joists and also seeps up into the underside of the deck lumber by capillary action within the wood fibres. Further it did nothing to provide for air flow around the timbers.